1) Field of the Invention
The present invention relates to particle shields, e.g. for preventing contaminant particles from reaching a mask. More particularly, the invention relates to the application of such particle shields in mask handling apparatus, mask storage boxes and/or in lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of the substrate.
2) Description of Related Art
The term xe2x80x9cpatterning meansxe2x80x9d as here employed should be broadly interpreted as referring to means which can be used to endow an incoming radiation beam with a patterned cross section, corresponding to a pattern which is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in the context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as here above set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC. This pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions, which are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate which is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book Microchip Fabrication: A Practical Guide to Semiconductor Procesing, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
In a lithographic projection apparatus, it is necessary to prevent any stray particles, which may be present in the apparatus from reaching, and becoming stuck to, the mask as they will then be imaged on the substrate and can be printed in the final device. Too high a level of contamination of the mask can lead to defective devices and the masks cannot generally be cleaned, or if cleanable can only be cleaned a limited number of times. In a lithographic projection apparatus using relatively long wavelength ultraviolet radiation, particles are prevented from reaching the mask by a pellicle. A pellicle is a thin membrane transparent to the radiation used in the projection beam of the lithographic apparatus and located parallel to but spaced from the mask. Contaminant particles moving towards the mask contact and stick to the pellicle. To ensure that the particles stuck to the pellicle are not printed on the substrate, the pellicle is spaced from the mask by a distance greater than the depth of focus at mask level.
However, it is not at present possible to provide a pellicle in a lithographic projection apparatus using UV radiation of 193 nm or 157 nm or extreme ultraviolet radiation for the exposure beam. Almost all materials are strongly absorptive of EUV radiation and a conventional membrane pellicle would have to be no more than about 30 nm thick in order not to cause unacceptable absorption of the projection beam. A membrane of this thickness would not have a sufficient lifetime in both a vacuum, during operation of the apparatus and atmospheric environment, during installation and service. Other stresses, such as optical stress and temperature variance would also likely destroy such a thin membrane very quickly.
An alternative approach to a separate pellicle membrane is to form a cap layer, again transparent to the exposure radiation, directly onto the mask. To be effective, the cap layer would need to be thicker than the depth of focus at mask level. The depth of focus at mask level is given by:                     DOF        =                              k            2                    ·                      λ                          NA              2                                ·                      1                          M              2                                                          (        1        )            
where xcex is the wavelength of the EUV radiation, NA the numerical aperture at wafer level, M the magnification of the projection optics and k2 a constant which is typically near 1. For EUV radiation of 13.5 nm, a numerical aperture of 0.25 and a magnification M of ⅕, the depth of focus at mask level is approximately 2.7 xcexcm. The effect of such a layer on an EUV projection beam would be excessive. The transmission, T, of radiation through a material with thickness d is given by:                     T        =                  exp          ⁡                      (                          -                              d                a                                      )                                              (        2        )            
where a is the attenuation length of the material (i.e. the length over which the intensity drops by a factor of 1/e). Even for a material which is relatively transparent to radiation at 13.5 nm, the attenuation length is about 0.6 xcexcm. Accordingly, a cap layer of thickness 2.7 xcexcm would absorb about 99% of all EUV radiation.
Furthermore, when shorter wavelength radiation is used for the projection beam, the sensitivity to contaminants is increased. At EUV wavelengths, a contaminant particle of only 50 nm diameter can lead to a faulty image. The need to keep the mask and other optical elements clear of contaminant particles is therefore extremely great.
Yet further, the projection beam of radiation in a lithographic projection apparatus may cause electrons to be freed from any surface on which it is incident. The surfaces which the beam of radiation is incident on include mirrors in the radiation system and the projection system as well as the substrate, sensors and the patterning means. Freed electrons in turn may break the bonds in water and hydrocarbon molecules present on the surfaces, resulting in reactive contaminants that cause damage to the surface. OH, in particular seems to cause substantial damage. Furthermore, the decomposition of the molecule does not absorb the stray electrons, which can then return to the surface and cause further damage on the surface.
It is an object of the present invention to provide a particle shield which is effective in mask handling apparatus and a lithographic projection apparatus using radiation of wavelength less than 200 nm, and especially extreme ultraviolet radiation, to prevent particles reaching the mask or any other component which requires protection from contamination, whilst avoiding unacceptable attenuation of the projection beam.
According to the present invention there is provided a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a devised pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of the substrate; characterized by
a particle shield for generating an electromagnetic field so as to prevent particles to become incident on an object to be shielded.
The particle shield may generate a substantially uniform (purely) electric field, generally transverse to the direction of particles approaching the shielded object, so as to exert a force on all charged particles which will deflect them away from the object to be shielded. Although such a uniform electric field may not deflect neutral particles, the radiation of the projection beam in a lithographic apparatus, which is the principle source of energy for airborne particles in a lithographic apparatus, is strongly ionizing so that any particles likely to cause problems will almost certainly be charged and will generally have a charge many times the charge of an electron. A substantially uniform electric field can conveniently be generated using a capacitor like arrangement of conductive plates.
The particle shield may, alternatively or in addition, generate a non-uniform electric field so as to induce a dipole moment in neutral particles and then attract those particles in addition to charged particles. A non-uniform electric field can conveniently be generated using a charged elongate member.
The particle shield may further generate an alternating, or other time varying field, instead of or in addition to the uniform or non-uniform static fields.
The particle shield may also, again alternatively or in addition to the uniform or non-uniform electric fields, generate a transverse radiation beam (i.e. oscillating electric and magnetic fields), or optical breeze, which will transfer transverse momentum to particles entering the transverse beam and absorbing photons from it. The radiation wavelength can be chosen so as to be absorbed by all expected particles but not expose the resist should any stray radiation reach substrate level.
The particle shield can also be a radiation source directing ionizing radiation, e.g. suitably short wavelength electromagnetic radiation or an electron beam, across the front of the object to be shielded. With such an arrangement the object to be shielded can be charged positively, to repel positively charged ions, compared to its surroundings, and/or relatively negative collection plates can be provided to attract positively charged ions. This arrangement ensures protection of the object to be shielded even when the main projection beam is off.
The object to be shielded is preferably a mask, since particles adhering to the mask are most detrimental to the quality of the projected image, but may also be a mirror or other element in the illumination or projection systems. Particles incident on, and possibly chemically reacting with, such elements may cause a loss in the reflectivity and therefore errors in the illumination dose received at the substrate.
By using electromagnetic fields rather than a physical barrier, the particle shield of the present invention performs its function without any attenuation of the projection beam.
In a further aspect the invention provides a mask handling device comprising a chamber for enclosing a mask during handling, transportation or storage thereof; and a particle shield for preventing or reducing the contamination of at least the patterned surface of said mask by particles. Said particle shield may comprise a means for generating an electromagnetic field so as to prevent particles to become incident on at least the patterned surface of said mask.
The present invention also provides a device manufacturing method comprising the steps of:
providing a substrate which is at least partially covered by a layer of radiation sensitive material to said second object table;
providing a projection beam of radiation using a radiation system;
using patterning means to endow the projection beam with a pattern in its cross section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation sensitive material; characterized by the step of:
generating an electromagnetic field so as to prevent particles to become incident on an object to be shielded.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, unless the context otherwise requires the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm) and EUV (extreme ultraviolet radiation, e.g. having a wavelength in the range 5 nm to 20 nm).